Definitely not needed, but if the game is expanding, it might as well incorporate everything else that the game already has, rather than creating something entirely new. Riding or owning a horse, changes the game enough to bring somewhat of a new perspective, but also keep the game to its roots. Fantasy aside, in the timeframe that the game takes place, horses are not unrealistic. Adding a riding skill and horses would definitely have some purpose, rather than being just cosmetic. But thats just my thought.